The present invention relates to a rolling bearing.
SUJ2, a kind of bearing steel, has been widely used as a material for races and balls which are parts of a rolling bearing. A large size rolling bearing employs SUJ3 or SUJ5 improved in hardenability.
To extend the service life of the rolling bearing, wear of a raceway surface or flaking from the raceway surface, called xe2x80x9cpittingxe2x80x9d, must be reduced. For this purpose, the bearing steel is subjected to hardening/tempering treatment for transformation to a tempered martenistic structure, thereby adjusting balance between hardness and toughness.
As an alternative approach to extend the service life, compressive residual stress is imparted to the raceway surface by a shot peening treatment. A technique for better use of the shot peening treatment is disclosed in, for example, an official gazette of Japanese unexamined patent publication 5(1993)-195069. According to this official gazette, attention is paid to residual austenite which is left untransformed during the hardening treatment. The bearing has its surface portion martensitically transformed by the shot peening treatment for accomplishment of the hardness while maintaining the residual austenite in its interior portion as it is, thereby accomplishing ductility.
Recently, however, bearings for automotive engine auxiliaries are used under more severe conditions so that a premature flaking phenomenon has manifested itself which is difficult for the conventional techniques to prevent. The premature flaking is a phenomenon where the raceway surface suffers rapid flaking due to an altered structure called xe2x80x9cwhite layerxe2x80x9d produced in the raceway surface. The premature flaking phenomenon is caused by high vibration and high impact load locally applying high shearing stress to the raceway surface.
As an approach to the prevention of the occurrence of such a white layer, proposal has been made to increase the viscosity of a grease base oil used for lubrication. However, this proposal has a problem of limitation on the lubrication method.
Another proposal has been made to make the material structure more stabilized or less susceptible to local deformation. For instance, it is proposed to reduce the amount of residual austenite by a subzero treatment because the residual austenite tends to decompose to promote the white layer formation. In this case, as well, the occurrence of the white layer is not effectively prevented.
In view of the foregoing problem, it is an object of the present invention to provide a rolling bearing adapted for effective prevention of the occurrence of the white layer.
In accordance with the present invention for achieving the above object, a rolling bearing comprising races and balls each constructed from steel is characterized in that a structure of at least a stationary-side race is transformed into a lower bainite structure through isothermal transformation and is adjusted for hardness to HRC=54 to 64, an amount of residual austenite to not more than 5%, and an area percentage of 0.8xcexcm or longer carbide to not more than 20% based on a total area of secondary carbide.
In the rolling bearing of the above arrangement, at least the stationary-side race has the lower bainite structure obtained through isothermal transformation. This results in a reduced amount of residual austenite as well as in uniform and fine dispersion of carbide particles dispersing operating stress and hence, the formation of the white layer is effectively prevented. As a result, the rolling bearing can be stably used over an extended period of time under conditions of high vibration and high impact stress.
Specifically, the inventors of the present invention have found that high shearing stress causative of the white layer can be more effectively dispersed in the lower bainite structure with more uniform and fine distribution of carbide particles as compared with the conventional tempered martensitic structure obtained by the normal hardening/tempering treatment. The inventors also found that the amount of residual austenite can be reduced by obtaining the lower bainite structure through isothermal transformation. Based on such findings, the inventors have accomplished the present invention.
The reason for adjusting the hardness to HRC=54 to 64 is because the hardness of less than 54 tends to encounter wear or deformation whereas the hardness in excess of 64 leads to reduced ductile-tough property and hence, to more likelihood of flaking.
The amount of residusal austenite is adjusted to not more than 5% for the purpose of preventing the aforementioned case where the residual austinite decomposes during the rolling motion, promoting the formation of the white layer.
The reason for adjusting the area percentage of 0.8 xcexcm or longer secondary carbide to not more than 20% is because if the area percentage of such secondary carbide exceeds 20%, the effect of dispersing the operating stress is reduced.
Incidentally, SUJ2 is preferred as steel for use in the above bearing. Where SUJ2 is used, the mechanical properties thereof are more effectively improved by the isothermal transformation treatment and an excellent resisting function to the white layer formation can be attained. Thus is provided the bearing of long service life which is more preferably applied under conditions of high vibration and impact stress.
Best modes for carrying out the present invention will herein below be described. However, it is to be noted that the present invention is not limited to these modes.